Loudness scaling revisited

This study investigated the likely value of measuring the loudness growth function in individual ears before fitting compression hearing aids. Examining loudness data from various published studies, we found that the variations in gain/compression prescription which might arise would most often be within the range of normal fine-tuning.

Background

During the 1990s a series of categorical loudness scaling procedures were proposed for the fitting of non-linear hearing-aids in subjects with sensorineural hearing losses. Given obvious procedural differences and a significant between-subject variance, the present work was undertaken in an attempt to evaluate whether the use of loudness scaling measures could provide useful information to the fitting process. Loudness scaling data from published studies were acquired and analysed statistically.

Methods

Normative data from seven different studies were collected and the normative references compared. It was demonstrated that the different scaling procedures relate the perceptual categories differently to sound level - e.g. at “comfortable” level a range of about 25 dB HL is found (see Figure 1). Normative data from three studies was used to evaluate the between-subject variance - e.g. at “comfortable” level a 95%-interval of about 35 dB HL is found.

Hearing-impaired data from four different studies were collected and analysed using a novel representation of the data, displaying the normalised, inverse slope of the loudness growth functions versus the hearing threshold level. In this representation the inverse slope varies linearly with the hearing threshold, and the observed variance becomes constant.

Results

We discovered that in 70-75% of the cases, the slope of the loudness function can be estimated from the hearing loss with an accuracy corresponding to a ± 5 dB fine-tuning of hearing-aid gain. The procedural differences for both normal and hearing-impaired subjects will result in loudness data that produce different input/output characteristics of a non-linear hearing-aid fitted to perform loudness restoration.

The significant between-subject variance in subjects with normal hearing, and an estimated correlation (r = 0.4) between the slope of the loudness functions in the individual with and (hypothetically) without a hearing loss, strongly suggest that the gain and/or compression ratio of a hearing-aid fitted to perform loudness restoration will be independent of whether the loudness function is measured or estimated.

Conclusion

On these grounds we concluded that the measurement of loudness scaling will generally not provide additional information for the fitting of non-linear hearing aids.

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Influencing today’s fitting software

The analyses contained in this study have contributed to the algorithms used in Oticon’s fitting software (Genie) to predict the loudness functions of clients based on the hearing threshold, thus improving the chances of a successful initial prescription.